An automatic steering system may steer a vehicle along a desired path. The steering system may use gyroscopes (gyros), accelerometers and a global navigation satellite system (a) to determine the location and heading of the vehicle. While steering along the desired path, the vehicle may need to stop. While the vehicle is stopped, the heading estimate will drift due to drift in the gyros.
When the vehicle starts moving again, the automatic steering system will have poor steering performance until the heading and roll estimations are corrected. If the heading is offset, the controller will try to correct this and if the roll is offset then the projection from the antenna position to the ground will be offset. These offsets will slowly be corrected for example by an extended Kalman filter. However, until the offsets are corrected the vehicle will not steer as precisely and have a wiggling behavior. In very low speed operations the estimation of heading is also challenged due to noisy and/or delayed heading information from a global navigation satellite system (GNSS).
A typical agricultural GNSS steering control system uses GNSS positioning and an inertial measurement unit (IMU) for heading information or uses a dual antenna to calculate heading based on the position of the two antennas. Due to crabbing, such as on a slope, the vehicle heading is not always aligned with the direction of the course over ground. GNSS also requires a good line of sight to satellites. Trees, buildings, windmills etc. can cause the GPS position to degrade or not be available. This is important for farmers that need precise vehicle control systems. Products on the market try to solve this problem by wheel odometry, inertial navigation systems (INS) and getting the best out of the available GNSS even though it has degraded, such as from real-time kinematic (RTK) fix to RTK float, etc.
Dual antenna systems may measure the heading and roll as long as there is high precision GNSS available independent of velocity. The extra antenna hardware however makes the system more expensive than single antenna systems. The precision of the heading is also limited by the length of the baseline between the two or more antennas and the precision of the GNSS signal. This can be a problem for certain vehicles, such as narrow vineyard tractors.
Single antenna systems rely on gyros and accelerometers to measure the roll and yaw of the vehicle. The yaw is used together with the GNSS course over ground to get a good a heading of the vehicle for control of the vehicle heading. Since the course over ground is not the same as the heading of the vehicle due to crabbing, a single GNSS system will not be able to directly measure the crabbing like a dual antenna GNSS system.
The roll and heading are also used for projecting the GNSS antenna position readings to the point on the vehicle to be controlled. Typically, the vehicle operator is concerned about the accuracy on the ground. The gyros and accelerometers drift over time and are especially affected by temperature, shocks and vibration, and depending on the technology and quality, also have a bias instability that is difficult to calibrate. These biases are compensated by the GNSS course over ground information based on the Doppler effect and/or low-pass filtered delta values between the last n position measurements from GNSS. Both course over ground sources from GNSS are poor at low speed and not available at a standstill.
As mentioned above, gyroscopes are used for navigation, guidance, and stabilization and/or pointing of many manned and unmanned systems designed for commercial, industrial, and military applications. From game controllers to smartphones, and from remote stabilized weapons to driverless vehicles, gyros and inertial measurement units (IMUs) perform a number of vital navigation, guidance, and positioning functions within these systems.
With the tremendous variety of applications comes an equally wide array of performance grades in gyros and IMUs. Consumer grade gyros such as those used in video game controllers, smartphones, tablets, and automobile airbag systems exist on the low-end of both performance and cost. More demanding applications such as weapons systems, driverless vehicles, and navigation in GPS/GNSS-denied environments require a much higher grade of performance. The performance capabilities and accuracy requirements determine which technology is integrated into a specific system.
Micro-electro-mechanical systems (MEMS) gyros offer smaller size and weight and less power consumption than other gyroscopes. MEMS are capable of withstanding high non-operating shock levels, and in general offer a lower cost than other gyro technologies. Some weaknesses of MEMS gyros and inertial systems lie in critical performance parameters such as higher angle random walk/noise, which is an extremely important performance criterion in stabilization and positioning systems. In addition, MEMS gyros have higher bias instability, which results in a degraded navigation or stabilization/pointing solution. Thermal sensitivity of MEMS gyros and inertial systems also impact their bias and scale factor performance. These attributes are important to both stabilization and navigation applications.